In general, when HFC-134a is produced from trichlene and HF as starting materials, the production process is effected by two reaction steps. That is, trichlene is allowed to react with HF as a first step reaction based on the following formula (1) to form 1,1,1-trifluoro-2-chloroethane (referred to as "CF.sub.3 --CH.sub.2 Cl" or "HCFC-133a" hereinafter), and then the thus formed HCFC-133a is allowed to react with HF based on the following formula (2) as a second step reaction to obtain HFC-134a. EQU CHCl.dbd.CCl.sub.2 +3 HF.fwdarw.CF.sub.3 --CH.sub.2 Cl+2 HCl (1) EQU CF.sub.3 --CH.sub.2 Cl+HF.fwdarw.CF.sub.3 --CH.sub.2 F+HCl (2)
Each of these reactions is carried out in the presence of an alumina-chromina catalyst, but under different reaction conditions: the first step reaction being effected under a pressure of 4 kg/cm.sup.2 G, at a temperature of 250.degree. C. and with an HF/trichlene mol ratio of 6/1; and the second step reaction is carried out under a pressure of 4 kg/cm.sup.2 G, at a temperature of 350.degree. C. and with an HF/HCFC-133a mol ratio of 4/1. The thus formed reaction products are separated by distillation which may be effected by various distillation systems. In any of the distillation systems, HCl is recovered as a by-product to be used for other purposes, and HF and HCFC-133a are recycled as reaction materials. HFC-134a as the product of interest is concentrated by the distillation and separated as a fraction containing small amounts of HCFC-133a and HF.
The presence of small amounts of HCFC-133a and HF in the HFC-134a fraction is attributable to the minimum azeotropic points of HCFC-133a with HF and HFC-134a with HF. That is, in the second step reaction under a pressure of 4 kg/cm.sup.2 G, the former mixture has an azeotropic point of 41.degree. C. at which its HCFC-133a/HF compositional ratio becomes 62 mol %/38 mol %, and the latter mixture has an azeotropic point of 14.degree. C. at which its HFC-134a/HF compositional ratio becomes 87 mol %/13 mol %.
Since HCFC-133a or HF cannot be removed from HFC-134a by distillation alone because of the above reason, HF is removed generally by way of alkali washing. Once HF is removed, HCFC-133a and other fluorocarbons can be separated from HFC-134a by distillation because they have no azeotropic relation with HFC-134a.
As described above, removal of HF from a concentrated HFC-134a fraction has been effected by alkali washing in the prior art. However, such a washing step not only requires an alkali agent but also entails discharge of the expensive HF and further causes a disadvantage because of the requirement of waste water treatment.